Conventionally in the manufacturing of metalized ceramic substrates for use as chip carriers, a negative photoresist subtractive circuitization process is employed. The metalization layer is typically a trilayer composed of a 1000 angstrom thick chromium layer, a 5-20 micron thick copper layer, and about a 1000 angstrom chromium layer which is disposed on a ceramic substrate. A negative photoresist, typically a polyisoprene cyclized rubber photoresist such as KTFR manufactured by Shipley Corporation, is applied atop the metalization, exposed, and developed to produce an image. The image is used for subtractive circuitization, in which the trilayer is etched in conveyorized etching equipment to form the surface circuitization; the metal that is not protected by photoresist is removed. First, a 45.degree. C. alkaline permanganate etchant is employed to selectively remove the top layer of chromium. Next, the copper layer is selectively removed by an iron chloride etchant. Next, the bottom layer of chromium is selectively removed by etching a second time with the 45.degree. C. alkaline permanganate etchant. After the etching is completed, the photoresist is stripped leaving a metalized pattern on the ceramic substrate. Portions of the remaining metalization may be removed, using similar procedures, during subsequent manufacturing steps.
The KTFR is a negative photoresist so it is not subject to the narrow processing windows to which positive photoresists are subject. Additionally, the KTFR photoresist can withstand high temperatures and can withstand both the iron chloride etchant and hot alkaline permanganate etchants. However, the KTFR photoresist requires halocarbon solvents, such as methyl chloroform (MCF) and Freon.RTM. to develop and strip the photoresist. Governmental regulations require that the use of such halocarbon solvents be eliminated within the next few years.
Acrylic based negative photoresists can withstand high temperatures and do not require MCF or Freon.RTM. for development. However, they demonstrate poor resistance to the 45.degree. C. alkaline potassium permanganate etchant. Imaged acrylic based negative photoresist degrade within about 15 seconds after coming into contact with the hot permanganate etchant. Also, such acrylic based negative photoresists are frequently undercut by the hot permanganate etchant.
It is desirable to have acrylic based negative photoresist that: can withstand both the iron chloride etchant and the hot alkaline permanganate etchant solutions; that demonstrates minimal undercutting; and that can be both developed and stripped by nonhalogenated reagents.